test: add unit test for inputExtensionDirs at filesystem root

Verify that inputExtensionDirs terminates when projectDir is "/"
instead of looping forever. Covers the fix for #14254.

Author: mcanouil

tests/unit/extension-dirs.test.ts

@@ -0,0 +1,26 @@
+/*
+* extension-dirs.test.ts
+*
+* Copyright (C) 2020-2026 Posit Software, PBC
+*
+*/
+
+import { unitTest } from "../test.ts";
+import { assert } from "testing/asserts";
+import { inputExtensionDirs } from "../../src/extension/extension.ts";
+
+// Test for issue #14254: inputExtensionDirs should not hang when
+// projectDir is the filesystem root.
+// deno-lint-ignore require-await
+unitTest("extension-dirs - no infinite loop at filesystem root", async () => {
+  const tempDir = Deno.makeTempDirSync({ prefix: "quarto-test-ext" });
+  const tempFile = `${tempDir}/test.qmd`;
+  Deno.writeTextFileSync(tempFile, "---\ntitle: test\n---\n");
+
+  // With projectDir = "/", the dirname loop must terminate at root.
+  // If the root guard is missing, this call hangs forever.
+  const dirs = inputExtensionDirs(tempFile, "/");
+  assert(Array.isArray(dirs), "inputExtensionDirs should return an array");
+
+  Deno.removeSync(tempDir, { recursive: true });
+});